The present invention relates to a polylactic acid resin composition and a method for molding the same, and more particularly, to a polylactic acid resin composition having an excellent crystallization rate and mechanical property and a method for molding the same.
Recently, efforts to reduce greenhouse gas have been extensively made against global warming, and development of a biodegradable polymer material degraded in nature has attracted attention as one of the efforts. For a typical polymer, petroleum resources are mainly used as basic raw materials but have a probability of exhaustion in the future, and carbon dioxide generated when the petroleum resources are consumed in a great quantity is considered as a main factor of global warming. Accordingly, the eyes are centered on development and industrial application of a biodegradable polymer using plant resources grown by using carbon dioxide from the air as raw materials.
Among biodegradable resins, a polylactic acid is a polymer using a lactic acid obtained by fermenting starch extracted from plants as a raw material, and a polymer which can be mass-produced due to excellent optical property, heat resistance, and cost balance among biodegradable polymers derived from biomass. The polylactic acid has excellent transparency and hardness as compared to another biodegradable resin, but has the largest drawback in that dynamic properties are poor at high temperatures. That is, when a temperature of the polylactic acid exceeds 60° C. that is a glass transition temperature of the polylactic acid, the polylactic acid is rapidly softened. A polylactic acid stereocomplex including a poly L-lactic acid (PLLA) constituted by only a typical L-lactic acid and a poly D-lactic acid (PDLA) constituted by only a D-lactic acid blended in a molten state is disclosed to solve the aforementioned limitation. The poly L-lactic acid has a spiral structure wound in a left direction, but the poly D-lactic acid has a spiral structure wound in a right direction. When two components are uniformly blended at high temperatures, a stereospecific bond is formed between the two components, thus forming a stereocomplex having a crystalline structure that is stronger than a crystalline structure formed in the case of only the poly L-lactic acid or the poly D-lactic acid. Consequently, the stereocomplex may have a high melting point and high crystallinity as compared to a homopolymer of the poly L-lactic acid or the poly D-lactic acid.
Meanwhile, heat resistance and impact resistance are reduced due to a slow crystallization rate during a molding process of a polylactic acid homopolymer or a stereocomplex resin composition. In review of related preceding documents, Korean Patent Application Laid-Open No. 2012-0108798 discloses a technology of setting a melting temperature to about 190 to about 195° C. in order to preserve a crystal of a stereocomplex of L-type and D-type polylactic acids having a crystallization rate improved through cooling after a cavity surface of a mold is heated to about 100 to about 110° C., but the melting temperature is low, and Korean Patent Application Laid-Open No. 2012-0108798 does not mention a nucleating agent suitable to improve the crystallization rate of the stereocomplex.
Further, Korean Patent Application Laid-Open No. 2012-0122815 discloses a polylactic acid resin composition including an L-type polylactic acid, a D-type polylactic acid, and phyllite powder in appropriate contents, and having improved crystallization rate, impact resistance, and heat distortion temperature, and mentions phyllite powder as the nucleating agent.
Further, Korean Patent Application Laid-Open No. 2012-0129500 discloses a polylactic acid composition having an improved mechanical property and crystallization rate by applying phyllite powder and carbon nanotubes which are subjected to organification surface treatment to a polylactic acid, and mentions phyllite powder and the carbon nanotubes as the nucleating agent for improving physical properties but not the stereocomplex as the polylactic acid.
Further, Korean Patent Application Laid-Open No. 2011-0067238 discloses a polylactic acid complex having improved compatibility and heat resistance, but does not propose specific application of a resin composition and a method for improving processability.